The invention relates to photodetector devices, especially semiconductor photodetector devices, and to a method of compensating for polarization dependent response of a photodetector of such a photodetector device. The invention is applicable to so-called xe2x80x9cpigtailedxe2x80x9d photodetector devices which have an integral optical waveguide, for example an optical fiber, for directing light onto a detection surface of the detector, and to so-called xe2x80x9cconnectorizedxe2x80x9d photodetector devices which have means for attachment of a separate optical waveguide.
Photodetector devices preferred for use in, for example, telecommunications systems, use semiconductor diodes, usually made of germanium or indium gallium arsenide (InGaAs). Where relatively long wavelengths are involved, such as in Dense Wavelength Division Multiplex (DWDM) systems, InGaAs detectors are preferred over germanium detectors because the former exhibit better temperature stability and more uniform sensitivity, especially in the longer wavelength spectral ranges of typical DWDM telecommunications systems. They also have a lower dark current. Both of these kinds of photodetector devices exhibit a response that depends upon the state of polarization (SOP) of the incident light. Unfortunately, however, InGaAs photodetector devices have a relatively high polarization dependent response (PDR) as compared with germanium photodetectors. For InGaAs photodetector devices, the PDR, i.e., the difference between maximum and minimum responses, typically is around 0.02 dB whereas, for germanium photodetectors, the typical PDR is about 0.005 dB. Consequently, when an InGaAs photodetector device is used to measure, for example, polarization dependent loss, the measurement accuracy is limited because of the relatively high PDR of the photodetector device.
An object of the present invention is to at least ameliorate the afore-mentioned problem of polarization dependency of photodetector devices which have a maximum response to incident light corresponding to a particular substantially linear SOP of that light.
According to a first aspect of the present invention, there is provided a photodetector device comprising a photosensitive detector formed from a material having a polarization dependent response having maximum (RMAX) and minimum (RMIN) values corresponding to substantially linear states of polarization of light incident thereupon that are orthogonal to each other, and at least one interface through which a light beam for detection passes before being incident upon a detection surface of the detector, the light beam traversing said at least one interface with a propagation direction that is not normal to the interface, the arrangement being such that said at least one interface is tilted about an axis substantially parallel to the direction of the linear state of polarization corresponding to said maximum (RMAX) value and by such a tilt angle that polarization dependent transmission introduced by the at least one interface will compensate at least partially for the polarization dependent response of the material of the photosensitive detector.
In one embodiment of the first aspect of the invention, the device has a window whereby, in use, the light is directed onto the detection surface, the detection surface and opposite surfaces of the window providing three said interfaces, cumulatively providing the compensation for polarization dependent response.
The photodetector device may have a connector part for releasably attaching to the device a waveguide for directing the light to said window along an optical axis (OA) of the waveguide, each of said three interfaces being at an acute angle to said optical axis.
The window may be fixed generally parallel to the detector surface and both tilted to the required degree, in which case three interfaces will provide the required compensation. Alternatively, the detection surface may be substantially normal to the propagation direction of the light when incident thereupon and opposite surfaces of the window provide two said interfaces that cumulatively provide the compensation for polarization dependent response.
The photodetector device may be mounted to a mount with its maximum response axis having a predetermined orientation relative to a reference surface of the mount. The mount may then be installed on or in the connector part so as to allow tilting of the mount about the maximum response axis or an axis parallel thereto. The connector part may be part of, or fitted to, equipment in which the photodetector device is to be used. This arrangement allows PDR compensation to be provided after the photodetector device has already been assembled.
Alternatively, the interface may comprise an angled end surface of a waveguide, typically an optical fiber, through which the light is directed onto the detector, either directly or by way of another interface provided by a said window or an intervening lens. In the latter case, the required polarization dependent transmission effect would be provided cumulatively by the interfaces.
In a further embodiment of the first aspect of the invention, the photodetector device comprises a photodetector, a lens and an input optical fiber for directing a light beam, in use, through the lens and onto the detection surface, the optical fiber having its end from which the light beam emerges secured relative to the lens with a lateral displacement relative to the lens optical axis so that the optical axis of the light beam when leaving the lens will be inclined relative to the lens optical axis by such an angle that polarization dependent transmission introduced thereby substantially corrects for polarization dependent response of the photodetector per se.
According to a second aspect of the present invention, there is provided a photodetector device comprising a photosensitive detector and a fiber waveguide for directing a light beam for detection onto a detection surface of the detector, the waveguide being fixed relative to the detector and having an end face facing the detection surface, the end face being inclined by such an angle that polarization dependent transmission effects introduced by the end face at least partially compensate for polarization dependent response of the detector.
According to a third aspect of the invention, there is provided a method of correcting for polarization dependent response of a photodetector device comprising a photosensitive detector having a maximum response (RMAX) corresponding to linear state of polarization of light incident thereupon via at least one interface between media having different refractive indices, the interface intersecting a propagation direction in which, in use, a light beam for detection will be incident upon a detection surface of the detector, the method comprising the steps of:
determining orientation of either or both of a maximum response axis and minimum response axis that correspond to states of polarization of incident light for which the response of the photodetector is a maximum or minimum, respectively; and
adjusting an angle between the propagation direction and the interface in a plane of the maximum response axis such that polarization dependent transmission introduced by the interface at least partially compensates for polarization dependent response of the detector.
The reference to a direction in which a light beam will, in use, be incident upon the detector is intended to take account of the fact that the light beam may be collimated or divergent. In each case, the light beam will be substantially symmetrical about its beam/cone axis which, preferably, extends substantially parallel to the optical axis and intersects the detector at or near its geometrical centre; otherwise the detector must be considerably wider than the beam spot.
The determination of the maximum response axis of the photosensitive detector may comprise the steps of directing a substantially symmetrical, polarized light beam onto the detector and monitoring a corresponding output signal of the detector; varying the state of polarization between a substantial number of possible states; identifying either or both of maximum and minimum values of the detector output signal; and, while maintaining that state of polarization which provided the maximum or minimum value, analyzing the light to determine the orientation of the maximum response axis of the detector.
Preferably, the light beam is collimated, though it could be converging or diverging.
The accuracy of the determination will depend upon the number of polarization states selected. While it would be possible to use the subset comprising linear SOPs, the polarization controller permits the selection of substantially all of the points on a Poincarxc3xa9 sphere. Such variable polarization controllers are well known.
While it is preferable to use a highly-polarized light beam, it is possible to use any light beam having a significant non-zero degree of polarization.
Accordingly, an alternative embodiment of the third aspect of the invention is a method of determining the maximum-response-axis response of the photosensitive detector comprising the steps of directing linearly polarized light onto the detector along the optical axis, repeatedly rotating the SOP of the linearly polarized light beam about the optical axis, tilting the at least one interface relative to the optical axis and about the maximum response axis, and determining said angle for which the difference between maximum and minimum responses of the photodetector is substantially minimal.
The subsequent determination of the required tilt angle may comprise the steps of directing linearly polarized light onto the detector along the optical axis, repeatedly rotating the SOP of the linearly polarized light beam about the optical axis, tilting the interface relative to the optical axis and about the maximum response axis, and determining the tilt angle for which the difference between maximum and minimum responses of the photodetector is substantially minimal.
Alternatively, the required tilt angle may be determined by the steps of: directing a polarized light beam onto the detector and monitoring a corresponding output signal of the detector; varying the state of polarization between a substantial number of possible states; and varying the angle of the interface to select an angle for which the difference between the maximum and minimum values of the output signal is substantially minimal, the selected angle being said tilt angle.
The photodetector device may be mounted rotatably in a mount while its maximum response axis is being determined and, once the maximum response axis is known, secured to the mount with the maximum response axis having a known orientation relative to a reference surface of the mount. The mount then may be used to install the photodetector into a holder or other part of, or for assembly to, the equipment with which the photodetector is to be used. Once the mount is installed, the tilt angle can be determined by directing a linearly polarized light beam onto the detector along said optical axis; repeatedly rotating the SOP of the linearly polarized light beam about the optical axis, tilting the mount relative to the holder and about the maximum response axis and measuring the output of the photodetector so as to determine the tilt angle at which the difference between maximum and minimum responses of the photodetector is minimal, whereupon the mount can be secured to the holder so as to maintain that tilt angle.
The interface may be at the surface of the photosensitive detector or at a surface of a window through which the light to be detected is incident upon the photosensitive detector. Such a window may provide two such interfaces.
In an alternative embodiment of the third aspect of the invention, the detector and window are fixed relative to each other in a housing of the photodetector device and the step of tilting the interface involves tilting of the housing, so that the window and the detector surface are tilted together. This method is suitable for use after the photodetector device has been manufactured.
According to a fourth aspect of the invention, a method of assembling a photodetector device according to such further embodiment comprises the steps of directing a polarized light beam onto the detector and varying the state of polarization of the light beam between a large number of possible states; adjusting the displacement of the optical fiber transversely relative to the optical axis of the lens while monitoring the difference between maximum and minimum values of the detector output signal; and fixing the optical fiber relative to the lens at a transverse displacement corresponding to the difference being a minimum.
Hence, in this case, it is not necessary to determine the maximum and minimum polarization dependent response axes of the detector beforehand.
Preferably, the light beam emerging from the lens is substantially collimated, though it could be converging or diverging.
The accuracy of the adjustment will depend upon the number of polarization states selected. While it would be possible to use the subset comprising linear SOPs, the polarization controller permits the selection of substantially all of the points on a Poincarxc3xa9 sphere. The variable polarization controller may comprise a polarization scrambler for randomly varying the SOP, or a controller which varies the SOP systematically. Such polarization controllers are well known.
While it is preferable to use a highly-polarized light beam, it is possible to use any light beam having a significant non-zero degree of polarization.
In one embodiment of the fourth aspect of the invention, the detector is mounted in a housing of the photodetector device and the lens is attached to the housing so as to direct light onto the detector through an opening in the housing. The lens receives the light from a fiber waveguide which then is offset laterally with respect to the optical axis of the lens to vary the angle at which the light impinges upon the detector surface.
According to a further aspect of the invention, there is provided a method of correcting for polarization dependent response of a photodetector device comprising a photosensitive detector (26; 96) having a maximum response (RMAX) corresponding to linear state of polarization of light incident thereupon via at least one interface (20xe2x80x2,20xe2x80x3,28,98) between media having different refractive indices, the interface intersecting a propagation direction in which, in use, a light beam for detection will be incident upon a detection surface (28; 98) of the detector (26; 96), the photosensitive detector having a photodetector axis (PDA) perpendicular to its detection surface, the method comprising the steps of:
with the photodetector device mounted rotatably in a connector part (16) with said photodetector axis (PDA) extending at an arbitrary acute angle (xcex8) to an optical axis (OA) of said connector part, directing polarized light onto the detection surface (28; 98);
while varying the state of polarization of the light successively between a substantial number of points on the Poincarxc3xa9 sphere, monitoring an electrical output signal from the photosensitive detector (26; 96) and registering the difference between maximum and minimum values thereof,
rotating the photodetector device (12) step by step using the photodetector axis (PDA) as a rotation axis, at each step, registering the difference between the maximum and minimum response and determining the rotation angle for which the difference is a minimum;
while maintaining said rotation angle constant, varying the state of polarization again through a substantial number of points on the Poincarxc3xa9 sphere, monitoring the output of the photosensitive detector (26; 96), adjusting the acute angle (xcex8) with respect to the optical axis (OA), in a step by step manner, and determining the acute angle at which the difference between the maximum and minimum values is a minimum; and
securing the photodetector device to the connector part (16) to maintain said rotation angle and said acute angle at which the difference between the maximum and minimum values is a minimum.
Typically, the acute angle is between 5 degrees and 8 degrees.
Embodiments of the various aspects of the present invention will now be described by way of example only and with reference to the accompanying drawings.